This invention relates to tungsten silicide layer and a semiconductor device and, more particularly, to a tungsten silicide layer used as a gate electrode and a wiring strip.
Tungsten silicide is attractive conductor. The tungsten silicide is much larger in conductivity than doped-polysilicon, by way of example. For this reason, the tungsten silicide is popular with semiconductor manufacturers, and is partially replaced in semiconductor devices as conductive layers such as, gate electrodes and wiring strips in order to reduce the resistance. The low-resistive tungsten silicide is conducive to the miniaturization of circuit components on semiconductor chips, and is expected to further lower the resistivity for semiconductor devices in the next generation.
In general, tungsten silicide is expressed as WSix, and the composition ratio x is usually offset from the stoichiometry at 2, i.e., WSi2, because the tungsten silicide with stoichiometric composition ratio of 2 is causative of compression stress in a layer overlain by the tungsten silicide layer. The composition ratio of the order of 2.6 is popular to conductive strips in semiconductor devices, and, accordingly, the tungsten silicide for the conductive strips are expressed as WSi2.6 as taught in Japanese Patent Publication of Unexamined Application (laid-open) No. 9-246206.
A sputtering and a chemical vapor deposition are usually used for growing the tungsten silicide. When a tungsten silicide layer is grown by using the chemical vapor deposition, a relatively small composition ratio Si/W is achieved, and low-resistive tungsten silicide WSix is grown through the chemical vapor deposition. However, the tungsten silicide grown through the chemical vapor deposition is liable to peel from the lower layer in a thermal oxidation at a later stage of the tungsten silicide growth. The peel-off is due to abnormal oxidation at the boundary between the lower layer and the tungsten silicide layer as reported in Japan Journal of Applied Physics, 1996, vol. 35, part 1, No. 2A, pages 584 to 588 and 1999, vol. 38, part 2, No. 2B, L209 to 211.
The prior art chemical vapor deposition is carried out on the following conditions. WF6 and SiH4 are used as the material gases. WF6 is reduced with SiH6, and tungsten silicide WSix is produced through the reduction. For this reason, the tungsten silicide WSix contains a non-negligible amount of fluorine atom. The fluorine atoms are the cause of poor adhesion and, accordingly, the peel-off.
Another chemical vapor deposition technique has been proposed. WF6 and SiCl2H2 are used in the prior art chemical vapor deposition. WF6 is reduced with SiCl2H2, and WSix is grown through the reduction. Although the resultant WSix still contains the fluorine, the amount of fluorine is decreased by three orders of magnitude. The difference in magnitude results in large adhesion to the lower layer, and, for this reason, the tungsten silicide is usually grown through the reduction of WF6 with SiCl2H2. However, if the composition ratio Si/W is still small, the tungsten silicide grown through the reduction of WF6 with SiCl2H2 is still liable to peel from the lower layer due to the abnormal oxidation at the boundary.
A solution is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 8-153802. According to the Japanese Patent Publication of Unexamined Application, the composition ratio x equal to or greater than 2.8 is effective against the peel-off due to the abnormal oxidation. However, such a large composition ratio x renders the tungsten silicide resistive. The highly resistive tungsten silicide makes the miniaturization of wiring strips in semiconductor devices difficult. On the other hand, the tungsten silicide layers liable to peel from lower polysilicon layers are not reliable, because the peel-off gives rise to increase of resistance along the salicide structure.
It is therefore an important object of the present invention to provide a tungsten silicide layer, which exhibits small resistivity.
It is also an important object of the present invention to provide a semiconductor device, which has highly reliable conductive strips formed of the tungsten silicide layer.
It is also an important object of the present invention to provide a process for fabricating a semiconductor device, in which the tungsten silicide layer is formed at a high reproducibility.
To accomplish the object, the present invention proposes to make tungsten silicide grains with  less than 001 greater than  orientation largest in volume ratio of all.
In accordance with one aspect of the present invention, there is provided a tungsten silicide layer incorporated in a semiconductor device, including tungsten silicide grains different in orientation having at least first tungsten silicide grains with  less than 101 greater than  orientation and second tungsten silicide grains with  less than 001 greater than  orientation having a volume ratio largest of all.
In accordance with another aspect of the present invention, there is provided a semiconductor device fabricated on a substrate, comprising at least one composite conductive path including a lower layer formed of silicon and an upper layer of tungsten silicide including tungsten silicide grains different in orientation and having at least first tungsten silicide grains with  less than 101 greater than  orientation and second tungsten silicide grains with  less than 001 greater than  orientation having a volume ratio largest of all.
In accordance with yet another aspect of the present invention, there is provided a process for fabricating a semiconductor device comprising the steps of a) preparing a substrate structure having at least a silicon layer, b) depositing a tungsten silicide on the silicon layer for producing a tungsten silicide layer, at least a part of the tungsten silicide layer at the boundary to the silicon layer being expressed as WSix where the composition ratio x ranges from 2.0 to 2.2, and c) treating the tungsten silicide layer with heat at 700 degrees to 850 degrees in centigrade so as to render the tungsten silicide layer including plural tungsten silicide grains different in orientation and having at least first tungsten silicide grains with  less than 101 greater than  orientation and second tungsten silicide grains with  less than 001 greater than  orientation having a volume ratio largest of all.